The present invention relates to a method for laying down a pavement consisting of paving material on a subgrade with a screed of a road paver in which a compaction unit of the screed pre-compacts the paving material in the course of cyclical work cycles having a selectable stroke and a selectable frequency while laying down pavement having a selectable pavement thickness at a selectable paving speed for road pavers; a screed for road pavers having a compaction unit with a tamper bar that is drivable in cyclical work cycles with a selectable stroke and a selectable frequency for pre-compacting a pavement made from paving material; and a road paver comprising at least one screed mounted on traction bars that are articulated to the road paver and the articulation points thereof are vertically adjustable with leveling cylinders and the screed comprising a compaction unit having at least one tamper that is operable with a selectable stroke and a selectable frequency.
When a pavement of bituminous or concrete-type paving material is laid down with a road paver, the floatingly towed screed should compact the paving material over the whole pave width as uniformly as possible and generate a continuous or closed flat structure. The compaction unit, e.g. a so-called tamper or a tamper and an eccentric vibrator, should generate a precompaction that is as high, uniform and constant over the pavement thickness as possible, so that different or varying pavement thicknesses have no significant impact on the final compaction. Stroke and frequency of the tamper influence the precompaction and floating behavior of the screed. The greater the stroke, the higher is the precompaction and the greater is the precompaction depth. The frequency can be adjusted individually in an infinitely variable way. EP 0 493 644 A discloses that e.g. the tamper frequency is adjusted in response to the paving speed. Furthermore, it is expedient when the tamper stroke is adapted to the pavement thickness such that the screed can perform paving with a positive setting angle that is as small as possible. If the stroke for the pavement thickness is too large, this may create a negative setting angle of the screed, possibly resulting in an open cracked surface structure or uncontrollable leveling behavior of the screed, with ensuing irregularities. The pavement thickness is e.g. predetermined by the setting of the height position of the traction points of the screed on the road paver. Likewise, the frequency and the paving speed must be matched with one another. So far the matching operation has been chosen individually such that the screed performs the paving operation at a positive setting angle that is as small as possible. On the other hand, the paving speed defines the action of the compaction unit on the surface. The paving speed must be chosen such that a material supply that is as constant as possible is ensured by the transport vehicles. Since the paving speed has a great influence on precompaction, it should be ensured that the screed performs the paving operation at a small positive setting angle so as to guarantee high evenness, i.e. the paving speed used must permit a high precompaction. The stroke has so far been set manually in several steps, with the paving operation having to be interrupted in each step. Each stroke step, however, just constitutes a compromise because it only fits one pavement thickness. For instance, a larger quantity of paving material is pre-compacted by the tamper bar due to an increase in stroke within the set pavement thickness. Precompaction can also be increased by increasing the frequency. In specific cases the tamper can cooperate with an additional eccentric vibration device in the screed so as to achieve even higher precompaction and evenness.
Starting from the information brochure “Für jede Aufgabe die richtige Einbaubohle” [“For each task the right screed”] of the company Joseph Vögele AG, 68146 Mannheim/Germany, No. 2400/10/2.1997, page 4, it is known that the stroke of the compaction unit including a tamper is adjusted manually in that an eccentric bush which is rotatable in a connecting rod driving the tamper bar is rotated relative to an eccentric section of the driving eccentric shaft. The eccentric bush is clamped on the eccentric section of the eccentric shaft and thereby coupled with the eccentric section in a rotationally fixed manner and can be rotated after release of a clamping screw relative to the eccentric section and can be fixed again. The eccentric shaft is driven by a hydromotor having a speed that is e.g. infinitely variable. If prior to the paving work a specific pavement thickness is set, the stroke is then adjusted to this pavement thickness. If the pavement thickness is changed, the paving work must be interrupted and the stroke must be adapted to the new pavement thickness. Since the pavement thickness can also vary during the ongoing paving operation by reason of external influences, the set stroke does often not fit the pavement thickness, whereby the precompaction varies and the setting angle of the screed can change and, as a consequence, evenness and surface quality of the pavement will deteriorate. The adjusting operation is time-consuming and troublesome for the reason that e.g. eight connecting rods may be provided in the base screed alone, and the adjusting operation must be carried out with great care to perform a uniform precompaction operation over the work width.
DE 198 36 269 A discloses a method for varying the frequency of the tamper in response to the setting angle of the screed, wherein the setting angle of the screed is continuously sensed via at least one sensor. The frequency is adjusted automatically whereas other machine parameters are set by an operator in response to the respective paving parameters.
DE 40 40 029 A discloses a method in which during paving the frequency of the tamper is varied depending on the actual paving speed. Other machine parameters are set by the operator as an additional measure. For instance, the stroke of the tamper must be set manually prior to paving or during an interruption of the paving work. This is tantamount to a considerable work load for the operator and calls for great expertise.